…cache key (#69)

`poetry.lock` contains charm dependencies and non-charm dependencies.
Only when charm dependencies change we need to create a new cache.

This PR replaces (in the cache key) `poetry.lock` with
`requirements-last-build.txt` which is generated by a tox `pack-wrapper`
environment.

`requirements-last-build.txt` is a misleading name in this context.
Normally, it would be generated after a `charmcraft pack` by running
`tox run -e build`. However, when we run `tox run -e pack-wrapper`, the
`charmcraft pack` step is skipped—so there was no "last build".

`requirements-last-build.txt` is identical to the `requirements.txt`
file that would have been used during `charmcraft pack`.

---

This PR is only a breaking change for charms that have implemented a
`build` tox environment which was added in #59. No dependent
repositories have implemented this, so this change will not break any
repositories. It will only break this PR:
canonical/mysql-operator#192

Will be used in future integration test reusable workflow

… pytest-operator `check_deps` (#72)

pytest-operator `ops_test` requires that charmcraft is installed:
https://github.com/charmed-kubernetes/pytest-operator/blob/9e0c8f3b4feb848e097391317c5a0a09d43bd3f4/pytest_operator/plugin.py#L229

…st (#73)

pytest_collection_modifyitems hook in plugin.py should run after tests
are deselected with `-m` (e.g. `pytest -m "not unstable"`) so that a
GitHub runner is not provisioned for an empty group

…in cached LXC containers (#74)

And add charmcraft snap revision to cache key

Potential workaround for
canonical/charmcraft#1202

Supports lxd; microk8s support planned in the future

…n` (#76)

`python` alias to `python3` added to self-hosted runners

https://chat.canonical.com/canonical/pl/fzegjtzsjffo9ks9tdbzen3ste

…PROXY`) to Juju machines (#77)

Fixes backup integration tests on self-hosted runners

…ro groups found (#78)